This invention relates generally to secondary air injection to combustion systems and, particularly, to dampers for secondary air tubes in fossil fuel fired boilers.
Combustion systems are used in numerous industrial environments to generate heat and hot gases. For example, boilers and furnaces burn hydrocarbon fuels, e.g., oil and coal, in stationary combustors to produce heat to raise the temperature of a fluid, e.g., water. Industrial combustors typically employ various burner elements to combust the fuel and air injectors to provide combustion air to ensure complete combustion of the fuel. A typical industrial furnace, whether gas or fossil fired and hereafter referred to as a boiler, typically includes a lower combustion zone and a generally vertically extending flue gas passage.
The air introduced into a combustion system may be staged. Primary air is mixed with the fuel as both are injected into a combustion zone. Secondary air (also known as overfire air) may be injected into a combustion chamber downstream (in the direction of flue gas flow) of the primary combustion zone. The secondary air may be used to burnout any unburned hydrocarbons remaining from the primary combustion zone.
Overfire air is typically injected into the flue gas at a location in the flue gas passage downstream of the combustion zone. The combustion air provided to the combustion zone may be reduced to suppress flame temperature in the combustion zone and NOx formation. Suppressing combustion temperature creates excessive unburned hydrocarbons in the flue gas. The overfire air, introduced above the primary combustion zone, completes combustion of the unburned hydrocarbons which are then converted to carbon dioxide and water.
In conventional boilers, the overfire air is introduced to the flue passage through injection ports in the front or side walls or both of the boiler. The amount of secondary air (overfire air) needed for effective burnout may vary depending on the operating condition of the combustion system. To adjust the amount of secondary air, dampers are closed or opened to vary the amount of secondary air flowing from the secondary air tubes into the flue passage. However, conventional dampers tend to either shut off secondary air flow or allow substantial amounts of air flow. Conventional dampers tend not to effectively allow for adjustable amounts of secondary air. There is a long felt need for an improved damper for a secondary (overfire) air system.